1. Field of the Invention
The present invention relates to an aqueous-based composition capable of forming a strippable, protective coating useful for protecting surfaces, such as glass, metal, ceramic, plastic or other materials of construction. More specifically, the compositions, and coatings formed therefrom, are particularly useful for protecting precision surfaces, such as optical lenses including but not limited to ophthalmic lenses, telescopic lenses, microscopic lenses, and the like, which protection is desirably only of a temporary nature.
2. Brief Description of Related Technology
In industries which handle workpieces having precision surfaces, such as the precision optical industry, an ongoing need exists for at least minimized, and desirably elimination of the, use of solvent in the manufacturing process. This need stems from the fact that such solvents are often organic-based and frequently flammable, hazardous from a health and general safety perspective, and/or present emissions and/or disposal concerns, which are subject to increasingly more stringent environmental regulation.
Two types of protective coating materials are generally employed in the precision optical industry. They are: conventional spray paint compositions and specialty coating formulations, both of which carry the concerns noted above.
The first type of protective coating, useful in connection with optical lenses and other precision surfaces, includes spray paint compositions, such as those commercially available from Sprayon Corp., under the trademark "KRYLON". Certain manufacturers of optical lenses use such spray paint compositions extensively as a protective coating for the finished first side of lenses during their manufacture. Such compositions are readily dispensed by a spray-on application and are thereafter dried quickly to provide a thin film on the finished lens surface.
One short-coming of such spray paint compositions as a protective coating (in addition to the flammability of the solvents and other combustible components employed as components thereof) is their permanence. That is, for removal purposes, surfaces which have been protected with spray paint compositions often require immersion in, or aggressive wiping with solvents, such as acetone, toluene, or trichloroethane. While the use of flammable solvents or ozone depleting chemical ("ODC") solvents in such spray-applied spray-on protective coating compositions has not been a significant drawback to small manufacturers of precision optical lenses i.e., since the overall quantity of solvents is used in low volume operations and is correspondingly small, volatile organic compounds ("VOC") emission will necessarily be low and thus regulatory violations for discharge ODC are less likely to occur!, industrial process facilities using large-scale (and thus high volume) solvent-based processes face the concerns noted above in a more pronounced and magnified manner.
The second type of protective coating employs solvent-based materials, a large degree of which are easily removable from the substrate surface after drying. Examples of such protective coatings include those based on polyvinyl acetate, mixtures of polyvinyl acetate and nitrocellulose resins, acrylic latexes and the like. See e.g., U.S. Pat. No. 3,404,488 (Cox).
The presence of organic solvents in formulated protective coating composition also presents VOC emissions issues and possible ODC compliance problems. Such compositions often also require a long drying time to accommodate the necessary solvent evaporation.
U.S. Pat. No. 5,061,518 (Langerbeins) speaks to and claims a method for providing temporary protection to the surface of a mechanical object which involves the use of an aqueous dispersion of an acrylic polymer. When the water is removed from the dispersion after application onto the mechanical object, a film of the polymer is formed which itself is reported to be removable when contacted with a combustible organic solvent.
U.S. Pat. No. 5,081,174 (Van Buskirk) speaks to and claims a water-based coating composition for forming a temporary, protective coating on metallic substrates which is removable with an alkaline aqueous cleaning solution. The '174 patent indicates that cross-linking of materials in the composition can impair the removability of a temporary protective coating. (Col. 4, lines 24-25.) Accordingly, the '174 patent is seen to avoid cross-linking reactions for the polymer of the composition.
U.S. Pat. No. 5,191,014 (Roberts) speaks to and claims an aqueous coating composition for use as a temporary coating which requires the presence of methacrylamide. The '014 patent specification indicates that "methacrylamide is necessary in order to obtain removal of the temporary protective coating by dilute alkali within a period of one minute and preferably the portion of the methacrylamide should be at least 5 weight percent in order to permit rapid removal of aged coatings." (Col. 11, lines 11-15.) The '014 patent specification also indicates that the coating compositions are "tolerant of neutralization by cations especially aqueous, ammonia or sodium." (Col. 6, lines 1-8.) The '014 patent further indicates that the dilute alkali is "alkaline water pH 12-12.5 containing surfactant and thickener . . . " (Col. 6, lines 41-43.)
International Patent Publication Number WO 90/08165 speaks to compositions for protective coatings which includes a mixture of nonvolatile and evaporant materials, such as defoamers, wetting aides, dispersing aides, plasticizers and coalescents, release aides, resins, thickeners and water. The '165 publication does not speak to a photoinitiated cure of such composition, and also does not speak to the cross-linking of its components to provide a cross-linked or polymerized reaction product.
Zeneca Resins makes available commercially under the tradename "NEORAD" NR-3709 a water-borne UV/EB curable aliphatic urethane dispersion which includes an acrylic monomer trimethylol propane triacrylate. In Zeneca product literature, the "NEORAD" NR-3709 dispersion is indicated to be UV curable, and in fact suggests the use of the commercially available photoinitiator "DAROCUR" 1173. However, the "NEORAD" resin is designed for the purpose of forming a permanent coating rather than a removable coating. Indeed, "NEORAD" NR-3709 is said in such product literature to have "excellent adhesion" and "solvent resistance". In fact, that product literature suggests a use for "NEORAD" NR-3709 as the basis for a UV curable wood lacquer which "has excellent resistance to water, solvents and plasticizers . . . ".
U.S. Pat. No. 5,380,387 (Salamon), the disclosure of which is hereby expressly incorporated herein by reference, describes radiation-curable lens blocking/deblocking adhesive compositions. The compositions of the '387 patent include a predominant amount of at least one radiation-curable acrylic-capped organic prepolymer resin, a lesser amount of a monofunctional ethylenically unsaturated diluent monomer, a minor amount of at least one non-reactive release agent, and a suitable photoinitiator. These compositions adhere the lens or lens blank to a support member or block, for subsequent machining, polishing and/or other necessary treatments.
Notwithstanding the teaching of the '387 patent, it would be desirable to have an environmentally-safe (e.g., aqueous-based, removable, protective film, particularly well-suited for use in the manufacture and subsequent handling and use of precision surfaces or other articles which are susceptible to degradation, contamination, or other actual or potential adverse effects in an unprotected state. It would also be desirable to have a composition that is useful for the temporary protection of the finished first side of an optical lens during the subsequent processing, e.g., grinding, polishing, and centering, of a second side.